AGeoJO wrote:
Are those images of the exact same exposure then? If they are, I guessed I didn't quite get how the noise reduction works by averaging several images exposed at the same exposure value... .

The predominate noise in an image is photon shot noise, which is random across multiple exposures, so if you average multiple exposures together you average out the noise.

If the scene is static and you're using a tripod to enable this technique, why not a low ISO exposure?
Astro shooting without a clock drive?
Edit: After reading the article linked, I see various possible reasons listed at the end. The fact that the 5D3 can do it in camera with jpegs probably won't be of use to me, but multi-exposure averaging in Photoshop is a useful tool to keep in mind. For some purposes, I think HDR and low ISO make more sense, but there are always exceptions. Thanks.

RobDickinson wrote:
I didnt realise the 5d3 can do this in camera?
Yes, with jpg output, though it can also save all the raw files.
It can even align hand held shots pretty well!RobDickinson wrote:
Longer exposure low ISO shots are subject to noise too...
Yes, and not generally helped by raising ISO to shorten exposure. Multiple exposure averaging, though, is routinely used in astrophotography, so I suppose there would also be occasional uses for it in other types of low light level shooting.

I used to do this with my 1D3, but using bracketing. I would take bracketed shots within a 1-2 stop window, and average them, the results were quite nice. Of course that is more along the lines of HDR methodology, but it helped with the noise as well.

RobDickinson wrote:
I assume that takes some tracking device though?
Yes, at that exposure length and focal length, tracking is necessary
An 8mm fisheye is limited to about 20 seconds before starts start to become elongated.

jchin wrote:
How do you do this? Do you need Photoshop?
It's a menu option on the 5D3.

For other cameras, you would have to do it in any image editing program that has layers, or an astrophoto image processing program (Iris, ImagesPlus, Registax, MaxDSLR, Astrostack, DeepSkyStacker aka DSS, MaxIm, Nebulosity, K3CCDTools, Pixinsight etc etc)

Yes and this is why it won't work for fixed pattern noise as that's not random. PS uses it's median filter to blend several images to reduce noise

Also the use of PS's median filter can also be used in a different way. Say you have a nice scene in front of you but there are people in the shot that spoil the image. Take several shots (the more the better) with the camera locked down on a tripod of the scene and as long as no one is stationary for all the shots you can eliminate them from the scene.

AGeoJO wrote:
...I didn't quite get how the noise reduction works by averaging several images exposed at the same exposure value.

As others mentioned, a lot of noise is random in nature, generated by the amplifier in the sensor, stray photons, etc., and so it appears in different places in the image, whereas actual image elements will be in the same position.

By stacking multiple images, the fixed data is increased in "density" by the number of images, but the random noise isn't.

If we were to shoot four exposures of a sixteen-block color array it might look like this, with the red blocks representing introduced random noise and the bottom image representing the combined "noise-reduced" image.

Pixel Perfect wrote:
Also the use of PS's median filter can also be used in a different way. Say you have a nice scene in front of you but there are people in the shot that spoil the image. Take several shots (the more the better) with the camera locked down on a tripod of the scene and as long as no one is stationary for all the shots you can eliminate them from the scene.

That can be done manually as easy as pie with any software that has layers and layer masks, like GIMP which is free and Free.